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Old March 13th, 2010, 05:44 PM   #1
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Lightbulb (not) BUSTED: The Nikon D200 really does(nt) cheat at its ISO ratings

BUSTED and confirmed, the D200 really does cheat by a full stop on its ISO speed ratings, and I'm guessing this might apply to the D40x, D60, D80, and the D3000 as well since they use the same 10MP CCD sensor.

I've always thought the D200 was a bit on the "slow" side as far as shutter speed vs indicated ISO. I first noticed this shooting some Kodak Portra 800 speed film indoors on my F100 with the Nikon 50mm f/1.4G lens. Swapping the lens over to my D200 I needed to crank the ISO up to 1600 to get the same shutter speed. Hmmm....

Another piece of evidence in very bright conditions. Over the winter I was shooting my D90 in full sun and bright snow, and at base ISO of 200 and max shutter speed of 1/4000s I was only able to shoot at a maximum aperture of f/4.5. The D90 was acting up so I ran into the garage to swap my 17-55 lens onto my D200 instead. ISO 100 vs ISO 200 I should be able to shoot at full aperture now at about the same 1/4000s shutter speed, right? Well I was able to shoot at the full aperture of f/2.8, but I noticed that the shutter speed was still down around 1/2000s and not 1/4k which suggested there was more than a 1 stop difference between the D90's ISO 200 and the D200's ISO 100, and possibly more like 2 stops. Hmmmm...

Well today I finally formally tested this back to back with the same 200mm f/4 AIS lens used on the two cameras, with a Gossen Digisix external meter being used as a referee.

Conditions: Steady overcast with some light rain, and 11.7 EV. Proper exposure thus should be 1/200s @ f/4 on a D200 at ISO 100, and 1/400s @ f/4 on a D90 with a base ISO of 200.

Here's the D200 at ISO 100, 1/200s, and f/4, but that isn't a proper exposure! That's underexposed by about a full stop, and the D200's own meter said so too. :smile:

Adjusted the D200 to ISO 200 with the same 1/200s and f/4 settings as before and now that's proper exposure for the scene, but the Gossen says at ISO 200 you ought to be getting a full stop faster shutter speed. So what's going on? Let's see what the D90 does now with the same 200mm f/4 AIS manual focus lens.

Now here's the D90 also at ISO 200 and f/4, but now you get the 1/400s shutter speed for proper exposure that the Gossen says you should be getting. Ignore the minor color and white balance differences. Oh, and ignore the -1EV exposure compensation in the D90 (in the EXIF). That was from when I was doing something else before. In full manual exposure mode with Auto ISO off that setting isn't doing anything, since it has nothing it can control. Ignore it and just stick to looking at ISO, shutter speed, and aperture.

Proper exposure with the D200 set to ISO 100 and f/4 is down at 1/100s and not 1/200s. The D90 at ISO agrees with the Gossen, and the D200 is definitely cheating by a full stop. Its ISO 100 is really ISO 50!

Another D90 shot for a second example. ISO 200, 1/400s, f/4.

The D200 set to the same ISO 200 and f/4 aperture as the D90, but still a full stop slower shutter speed to get the same exposure. This is 1/200s and not 1/400s!

So the conclusion, the D200 actual ISO is a full stop slower than indicated.

Indicated ISO 100 = actual ISO 50
Indicated ISO 200 = actual ISO 100
Indicated ISO 400 = actual ISO 200

Another piece of evidence. Ken Rockwell in his ISO 3200 test of the D3, D300, D200, and D90 also noted this difference stating that he needed a shutter speed of 1/250s on the D200 to get the same exposure that the others got at 1/400 to 1/500s, so it holds at high and low ISO and bright and dark conditions. Google that one if you want to. I've seen this mentioned by some folks in passing at Nikon Cafe.com too.

Anyhow, I actually like this. I've always loved how easy it is to shoot the D200 at very large apertures even in bright conditions without needing ND filters. Even in the brightest sun with snow reflections you can still get f/2 or even faster no problem. I've never tried it, but you could probably shoot at f/1.4 and 1/8000s in the brightest conditions there are. ND filters in these conditions might cause contrast loss and ghosting. Maybe this explains why the D200 ISO 100 is so phenomenal also? I knew before buying the D200 that it was never going to work well at high ISO, which is why I eventually picked up a D90 too to handle all of the work up top.

If you check the histograms on the shots above they should all be pretty close. Conditions were steady at 11.7 EV throughout the shoot, but might have varied by +/- one-third of a stop here and there. But I'm spotting a full stop difference here and it was obvious that the D200 was off. So any minor exposure variations here would be tracking with conditions and not the cameras.

Flickr set here: http://www.flickr.com/photos/spake77...7623612288718/
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Old March 13th, 2010, 07:28 PM   #2
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nice research!
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Old March 15th, 2010, 03:33 PM   #3
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Yes, maybe. The theory is now being picked apart elsewhere.

The aperture could be off. No. That's why I tested with the same exact lens on two cameras. Through that lens set to f/4, what you get is what you get whether it's really f/4.5 or f/3.5, but it's the same amount of light transmission on both cameras, which "should" result in the same exposure at a given ISO sensitivity on each camera, but it wasn't. If both cameras were slow by a full stop then yes, I'd suspect something with the lens, but only one was off.

The shutter could be off. Highly unlikely, because the camera gives perfect exposures in the auto exposure modes almost everytime, which tells you that the meter and the shutter are in perfect sync with each other. It's also basically a new camera, bought in 2009 and still has less than 20,000 shots on it which is nothing. On really old cameras you might have a "slow" shutter which could result in overexposure, but in this case it'd have to be shooting "fast" by a full stop.

Active D-Lighting can affect exposure. While that may be true and I did have mine set to Auto on the D90, what would it have been controlling? ISO was set to full manual. Shutter speed was set manually. Non-CPU lens so the D90 didn't even know what was attached to it as far as aperture goes. It was manually set to f/4. It's worth it to do another back to back test with ADL turned off on the D90, but I think I'm going to see the same exact thing.

All cameras vary over time, it might just need adjustment. Yeah maybe, but I've never heard of exposure going off by a full stop even on the very first DSLRs like the D1. A 1 year old D200 with less than 20,000 shots that still gives perfect exposures in the auto exposure modes needing adjustment? Everything works perfectly with the camera and always has. The only thing that appears to be off is what the real world calls ISO 100 sensitivity and what Nikon is calling ISO 100 sensitivty on the D200. The Japanese are well known to fluff up and cheat on their specs as much as they can get away with, and this might just be another example.

Task list:

- Do a back to back shot on the D90 with the 200mm f/4 with Active D-Lighting set to Auto and then turned OFF to make sure it wasn't affecting anything, or was even working.

- Compare shutter speeds of the D90 and D200 and make sure they both give about the same amount of motion blur on an object with a constant amount of motion (think ceiling fan). I have something at work I can test like this, but I might not be able to do it at the same shutter speeds I was working with, but if they're in sync at one speed they should be at all speeds is my guess, so whatever I come up with should work.
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Old March 16th, 2010, 04:33 PM   #4
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Yes indeed, it looks like Active D-Lighting was the culprit.

D90 Active D-Lighting Auto Left, Off Right, same exact exposure settings

That was with the same 200mm f/4 AIS non-CPU lens, so yes indeed, Active D-Lighting is fully awake and messing up your shots even with a non-CPU lens. Looks like it performed a good two-thirds of a stop push in this particular shot when set to Auto.

Then a back to back with the D200.

D200 left, D90 right, both set at 1/200s, ISO 400, f/4, shot in RAW, opened in ACR 5.5 (PS8) and saved straight to JPEG with no changes or PP. Too dark to try any ISO 100 tests.

That's just one set of many. The D90 is just the tiniest bit brighter, maybe not even 1/3EV. To minimize time between shots and also reduce lens changing, I shot with my 80-200mm f/4 AIS zoom on the D200 and the 200mm f/4 AIS prime on the D90. Later in back to back lens-only tests on the D90 I found that the prime had just the slightest bit more light transmission at f/4 than the zoom, which would explain the D90 getting slightly more exposure.

So it looks like I'm the one that's busted.

As for why the D200 initially came up under-exposed by about a full stop, I have no idea. I must have gotten a false meter reading or something, or I suppose it's possible that there was just a little bit less light at the target location than from my deck which is where I took the ambient light reading. Technically I should have taken a reflected light reading off of the target itself instead of an ambient so that could have been a factor. So the D200 was left under-exposed, but since I had left ADL to Auto on the D90 I guess it did an in-camera push of about two-thirds of a stop to try and get a proper exposure. And I think I now know the source of my random unexplained over-exposure with this camera too. I think I'll try shooting with ADL turned off on this thing for awhile.

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Old March 25th, 2010, 08:45 AM   #5
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I hate Active D-lighting. I loved it at first, but then I realized it really takes away from nice contrast.
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